THE TIME BETWEEN FIRST RADAR ECHOES AND FIRST VHF LIGHTNING

RADIATION SOURCE LOCATIONS AS AN INDICATOR OF EVENTUAL STORM

INTENSITY

Patrick T. Hyland, William H. Beasley, Stephanie A. Weiss

School of MeteorologyUniversity of Oklahoma

Norman, Oklahoma

Rationale

The time between the appearance of the first radar echoes above some threshold and the occurrence of the first few VHF source locations nearby as determined by an LMA ought to be in some measure related to the strength of convection.

If the time is short, it stands to reason that the convection is strong and therefore, all else being equal, the storm would likely eventually be more intense than if the time had been longer.

Question

How does one measure storm “intensity”?

Chose to avoid use of the term “severe”

Chose to avoid properties that are difficult to measure, e.g., manifestations reported in “Storm Data”

Chose properties that could be determined from radar and NLDN

Data

WSR-88D Level-II Base Reflectivity (KTLX)First echoesLater storm development

VHF radiation source locations from OKLMA

Lightning ground-strike locations from NLDN

Limited to discrete, convective storms

Definitions

First radar echo:

30 dBZ echo at 0°C isotherm

First VHF source points:

10+ points in immediate vicinity of first echo

Procedure

Looked at spatial and temporal relationships among radar, VHF sources and CG ground strike data Three seasons

Winter (December) Spring (March) Summer (August)

NSSL WDSS-II

Considered several possible indicators of intensity Maximum tops (MT) height difference between MT and EL , MT and FL number of CG lightning flashes core radar reflectivity increases

Determined time intervals between first radar echoes and first VHF source points

Compared time intervals with chosen intensity indicators

Also looked at time between first VHF source points and first CG

11:30:36 11:35:29

11:36:02 11:37:28

Number of cases having given time between first echo and first VHF source locations (August)

Number of cases having given time between first echo and first VHF source locations (March)

Number of cases having given time between first echo and first VHF source locations (December)

Average Number of CG locations per storm case having given time between first echo and first VHF source locations

Average Number of CG locations per storm case having given time between first echo and first VHF source locations

Average Number of CG locations per storm case having given time between first echo and first VHF source locations

40/52 = 77%

Conclusions

For more than half of cases the time between the first radar echo and the first LMA source points was 15 minutes or less.

There was a tendency for the MT in each category to decrease with increasing time between first radar echo and the first LMA source points.

There was no discernible pattern in the height differences of all the echo tops relative to the EL or FL as a function of time from radar to vhf sources.

Average number of CG lightning strikes per storm was ambiguous indicator.

For storms for which the time between the first radar echo and the first LMA source points was short, it was more likely that radar reflectivity would increase at least 5-10 dBZ following the first CG lightning strike.

For 70% of cases, the time between first VHF source points and first CG lightning location was 5 minutes or less.